The discovery poses a further setback to efforts to contain the nuclear crisis as workers find themselves in increasingly hazardous conditions.

In another new finding, Tokyo Electric Power Co., which runs the Fukushima Daiichi Nuclear Power station, said late yesterday that it had detected an increase in levels of plutonium in soil samples taken from within the compound a week ago, raising fears of yet another dangerous element that may be escaping the crippled reactors.

It was unclear where the plutonium had come from. The reactors could be a source, but tests of nuclear weapons in the atmosphere, which ended in 1980, left trace amounts of plutonium around the world.

The highest levels in the soil, of plutonium 238, were found about 500 yards from the most heavily damaged reactors, the company said. It said lower levels of plutonium 239 and 240 had also been found, at amounts not significantly higher than normal.

All the reported readings are within the safe range of plutonium levels in sediment and soil given by the US Agency for Toxic Substances and Disease Registry. But Tokyo Electric said the highest reading was more than three times the level found in Japan compared with the average over the last 20 years.

American nuclear experts expressed confusion yesterday about the company’s latest report that one form of plutonium was found at elevated levels at the Fukushima plant while other forms were not, and suggested it could be a measurement error.

The contaminated water threatening the ocean had radiation measuring 1,000 millisieverts per hour and was in an overflow tunnel outside the plant’s Reactor No. 2, Japan’s nuclear regulator said at a news conference.

The maximum dose allowed for workers at the plant is 250 millisieverts in a year. Exposure to 1,000 millisieverts for 30 minutes could trigger nausea, and four hours of exposure might lead to death within two months, the US Environmental Protection said.

The affected overflow tunnel leads from the reactor’s turbine building, where contaminated water was discovered on Saturday, to an opening just 180 feet from the sea, said Hidehiko Nishiyama, deputy director-general for the Nuclear and Industrial Safety Agency.

The contaminated water level is about 3 feet from the exit of the vertical, U-shaped tunnel and rising, Nishiyama said.

Contaminated water was also found at tunnels from the No. 1 and No. 3 reactors, though with much lower levels of radiation.

“We are unsure whether there is already an overflow’’ of the water out of the tunnel, Nishiyama said. He said workers were redoubling efforts to remove the water from the Reactor No. 2 turbine building. Government officials have said that the water is probably leaking from pipes inside the reactor, from a breach in the reactor’s containment vessel, or from the inner pressure vessel that houses the fuel.

The nuclear safety agency reported that radioactive iodine 131 was detected Sunday at a concentration 1,150 times the maximum allowable level in a seawater sample taken about a mile north of the drainage outlets of Reactor Nos. 1 through 4. It also said that the amount of cesium 137 found in water about 1,000 feet from the plant was 20 times the normal level.

Nishiyama said there were no health concerns, because fishing would not be conducted in the evacuation-designated area within about 12 miles of the plant, the Kyodo news agency reported.

The disclosure about the escaping contaminated water came as workers pressed their efforts to remove highly radioactive water from inside buildings at the plant. The high levels of radioactivity have made it harder for them to get inside the reactor buildings and control rooms to get equipment working again, slowing the effort to cool the reactors and spent-fuel pools.

Workers pumped less water into the reactors yesterday in an effort to minimize the overflow of radioactive water from them, slowing the cooling process, Tokyo Electric said.

Although the source of the plutonium found at the plant was unclear, all three kinds of nuclear fuel at the complex could leak plutonium. Reactor No. 3 is fueled partly by mixed oxide fuel, or mox, which is made from plutonium and uranium. The fuel for most reactors is uranium.

But plutonium is a regular byproduct of a reactor’s splitting uranium atoms in two. Some of the speeding subatomic particles of the fission process turn uranium into plutonium.

So fuel rods that undergo fission get riddled with plutonium, though less than in mox fuel. Thus any of the reactors at Fukushima Daiichi could leak plutonium, as could spent fuel rods in cooling pools atop the reactor buildings.

The most abundant type of plutonium, the 239 isotope, has a half-life of 24,000 years and emits alpha rays. If deep inside the body, alphas can cause healthy tissue to turn cancerous. But the rays are so weak that outside the body they can be stopped by skin or tissue paper.